The present invention relates to a process for the preparation of trimethylsulfoxonium salts. Trimethylsulfoxonium salts have a number of uses within the chemical industry, and are particularly useful when utilized as the trimethylsulfoxonium salt of N-phosphonomethylglycine which is an effective herbicide. The salts are not easy to produce. One method disclosed in the prior art, U.S. Pat. No. 3,534,105, involves reacting a sulfonium salt with aqueous hydrogen peroxide in the presence of an inert solvent at a temperature of from 20.degree. to 100.degree. C. Efforts to duplicate the process disclosed in that patent have not been successful. Another method of manufacture involves the reaction of dimethyl sulfoxide with methyl iodide, as described by R. Kuhn and H. Trischmann, Ann., 611, 11721 (1058). The disadvantage of this method is that it involves the use of expensive methyl iodide and the reaction is very slow. Still another method involves the reaction of dimethyl sulfoxide with methyl bromide, this reaction being described in U.S. Pat. No. 4,141,920. The disadvantage of this reaction is that it requires greater than atmospheric pressure, the reaction is slow, and explosions during the reaction have been reported. Consequently alternative methods of producing trimethylsulfoxonium salts are continually being sought. The present invention relates to one such alternative method of producing the salts.